OBJECTIVE To understand the hormonal and behavioral mechanisms leading to quality infant caretaking in cooperatively breeding monkeys. RESULTS We have found that male caretakers lose significant weight while carrying infants with weight loss being inversely related to the number of helpers, providing the first documentation that there are costs to male infant care and to the value of multiple helpers. We have completed study of three pregnancies of five experienced parents and 5 new parents to examine hormonal changes during pregnancy and parturition, and changes in behavioral interactions between mates. These results are compared with infant survival and the quality of infant care provided. We have evidence that multiple caretakers buffer infants against extreme variability in caretaking of any one care giver. DISCUSSION We are halfway through a longitudinal study of ten family groups over three pregnancies to examine the causes and consequences of variation in infant care taking to determine how animals are recruited to help care for infants, and what benefits there are in terms of infant growth, development and behavior of multiple care givers. Results will have relevance to human families with two working parents. FUNDING MH35,215, MH 00,177 PUBLICATIONS Achenbach, G. G. & Snowdon, C. T. 1998. Response to sibling birth in juvenile cotton-top tamarins (Saguinus oedipus), Behaviour, 135:845-862.